With many types of film formation processes, it is not a problem to produce the films with the components in the desired proportions, for example in the formation of silicon dioxide or silicon nitride in silicon-based integrated circuits. However, in much of this conventional process technology, some variation in the exact stoichiometric proportions is tolerable without adversely affecting the performance of the resulting integrated circuits.
The task of forming thin films, such as those used in integrated circuits and other applications, is made more difficult when the film is formed of a multiplicity of components, for example, more than two. Such thin film formation processes have particular problems in the fabrication of superconducting material films and other types of films requiring exact stoichiometric proportions, as might be applied to integrated circuits, printed circuit boards, interconnect structures and other substrates. For example, a major difficulty experienced in the fabrication of the recently discovered superconducting ceramic compound YBa.sub.2 Cu.sub.3 O.sub.7 is adequate oxygen incorporation into the film during deposition or formation.
Another problem is that sufficient oxygen does not always remain a constituent of the film if the ceramic thin film is subsequently improperly annealed after formation. Annealing has been found to sometimes reduce oxygen content. Nevertheless, in some processes, annealing is required to form orthorhombic crystal structures needed for high critical temperatures, T.sub.c , that is, the temperature below which the superconducting characteristic is observed.
Many different types of techniques for the production of the older, lower T.sub.c superconducting films are known. For example, processes for electrodeposition of materials on a substrate using liquid electrolytes for forming superconducting films at low temperatures, called "cryoelectrodeposition", are known. Anodizing thin superconducting films to improve their properties is also known.
Another known technique for forming layered, multiconstituent films involves a low temperature method using at least two ballistic particle streams that are caused to intersect in a volume of space proximate to the substrate. One particle stream, the "gas" stream, comprises excited neutral particles, and the other particle stream, the "metal" stream, consists substantially of a particle species capable of chemically reacting with the excited neutrals. The excited neutrals are typically produced in a RF-generated plasma or by means of photon excitation, while the source of the metal stream is typically an evaporator.
One method for avoiding interlayer contamination of superconducting structures, which can degrade the quality of the devices, is to deposit two or more layers within an ultra-high vacuum (UHV) chamber without breaking vacuum between depositions. The depositions themselves present relatively few difficulties since independently controlled multiple evaporators or sputtering sources can be provided inside the UHV chamber. However, these techniques are not helpful in producing multi-component films homogeneously since they tend to form separate layers of elements, rather than one layer of the correct, mixed stoichiometric proportion.
The sputtering of a ceramic target of an oxide superconducting material, such as BaPb.sub.1-x Bi.sub.x O.sub.3 (0.05.ltoreq.x.ltoreq.0.3), for example, for forming superconducting electrodes is further known.
A surface reaction process for controlled oxide growth is also known which uses a directed, low energy ion beam for compound or oxide formation. In contrast with RF oxidized junctions, such ion beam oxidized junctions contain less contamination by back sputtering, and the quantitative nature of ion beam techniques allows greater control over the growth process.
In the production of the new, high T.sub.c ceramic superconducting films, many workers who have demonstrated superconducting thin films have used electron beam co-evaporation in a vacuum into which oxygen was introduced at as high a pressure as tolerable by the evaporator's vacuum system. This technique is marginal with respect to incorporating enough oxygen into the deposited thin films for a number of reasons. First, the evaporation produces "droplets" or clusters of the metals, for example yttrium, barium and copper, and limits the bonding of interior atoms to the oxygen in the vacuum. Secondly, it is difficult to produce films of repeatable stoichiometry using co-evaporation from multiple, separate hearths; for example, three hearths if yttrium, and copper are used. Other elements useful in the production of superconducting films include lanthanum, niobium, lead, strontium, aluminum, thallium, barium and mixtures thereof, among others.
Chemical vapor deposition (CVD) is impractical for forming these types of superconducting thin films since copper does not readily form volatile species. The use of organo metallic chemical vapor deposition (OMCVD) has recently been applied to superconducting materials, but only resulted in making low critical current superconducting films. The critical current, I.sub.c, is the current above which the superconducting state ceases.
RF sputtering from a composite target does not afford process flexibility for tuning the superconducting film characteristics. Only films with low critical currents have been produced this way.
It has also been found that the as-deposited film characteristics are highly dependent upon the reactor configuration. Single pass, multi-target, or even several-pass multiple target reactors, such as those used by MRC and Hypres, are of limited use inasmuch as only structures of multiple or alternating layers of discrete components may be formed, rather than homogeneous crystalline compounds.
Epitaxial regrowth techniques, such as those employed by IBM with respect to strontium titanate substrates have provided results at great expense. Ion cluster beam (ICB) deposition systems are known, but do not include the process and equipment modifications discovered in conjunction with the present invention.